How did the last subnet become useable?

alejob

I'm having trouble understanding the use of the last subnet.

When you start the curriculum they you should not use the first and last subnets(because the first is the network address and the last is the broadcast address).

Now in semester 3 you learn that you can actualy use this subnets. They say that if you use the command ip subnet-zero you can use the first subnet, which is fine and dandy. But they don't give any explaination on the last subnet. How did it became usable? I hated when I have to do something(like using the last subnet) just because "thats the way it is".

Can someone explain this to me? Has it always been useable and cisco just didn't want us to know?

sab4you

its useable but for the test purposes you will always state the first and last are not useable.

forbesl

Check this link out for an explanation:

http://www.cisco.com/warp/public/105/40.html

JSW77

It all has to do with whether or not the subnet mask information is sent. If you are using routers and a protocol that DO include the subnet mask in routing updates, then you can use the two previously unusable subnets, because the subnets can be distinguished from the network address and broadcast address. (in other words, if you ARE using a protocol and routers that let you include the subnet mask information in routing updates, the addresses to an address in a subnet will be sent as, say, 172.16.0.0/26 (a subnet), not just 172.16.0.0 by itself (the router doesnt know if this is the network address or a subnet, so it treats it as the network address). Its all about whether or not the subnet mask information is included in the routing updates.

PS- i havent studied cisco in quite a while, so i hope im correct about this, will someone please correct me if im wrong?! (nice vote of confidence on my part huh?

Webmaster

JSW77 wrote:

It all has to do with whether or not the subnet mask information is sent. If you are using routers and a protocol that DO include the subnet mask in routing updates, then you can use the two previously unusable subnets, because the subnets can be distinguished from the network address and broadcast address. (in other words, if you ARE using a protocol and routers that let you include the subnet mask information in routing updates, the addresses to an address in a subnet will be sent as, say, 172.16.0.0/26 (a subnet), not just 172.16.0.0 by itself (the router doesnt know if this is the network address or a subnet, so it treats it as the network address). Its all about whether or not the subnet mask information is included in the routing updates.

PS- i havent studied cisco in quite a while, so i hope im correct about this, will someone please correct me if im wrong?! (nice vote of confidence on my part huh?

I'm sorry, but no that is not correct. What you describe (subnetmask included in routing updates) is the requirement for VLSM.

forbesl

JSW77 wrote:

It all has to do with whether or not the subnet mask information is sent. If you are using routers and a protocol that DO include the subnet mask in routing updates, then you can use the two previously unusable subnets, because the subnets can be distinguished from the network address and broadcast address. (in other words, if you ARE using a protocol and routers that let you include the subnet mask information in routing updates, the addresses to an address in a subnet will be sent as, say, 172.16.0.0/26 (a subnet), not just 172.16.0.0 by itself (the router doesnt know if this is the network address or a subnet, so it treats it as the network address). Its all about whether or not the subnet mask information is included in the routing updates.

PS- i havent studied cisco in quite a while, so i hope im correct about this, will someone please correct me if im wrong?! (nice vote of confidence on my part huh?

You should check out the link I posted above, also. It will give you a good understanding.

Webmaster

forbesl wrote:

You should check out the link I posted above, also. It will give you a good understanding.

I agree that page says it all (I posted that link several times before...). Subnetting according to RFC 950 is also known as traditional subnetting (still subtracting the 2 subnets in the calculation).
RFC 1878 states this practice is obsolete, however the main question is of course which one you should use for your exams (in reality it simply depends on whether all routing network devices support it.). It's been a year since I took my last Cisco exam (BSCI) so I don't know how it is for the nex CCNA exam for example, but before RFC 1878 you would get some subnetting questions which included the correct answer (the one where the 2 subnets were subtracted) and an incorrect answer (where the 2 were not substracted. When RFC1878 became active, Cisco could, of course, not use the same questions because it would be too tricky. So what they did instead are two things:
- not include an incorrect answer/choice that would be correct depending on which RFC's subnetting method is used.
- or more common: they made it obvious when the question required you to use the modern subnetting method, by mentioning that the ip subnet-zero command is enabled.


Here's another good page at Cisco regarding subnetting.

www.cisco.com/en/US/tech/tk365/tk554/technologies_tech_note09186a00800a67f5.shtml